SOLAR RADIATION 



213 



In computing the atmospheric depletion of solar radiation as shown 

 in Fig. 1, the effect of the varying distance of the earth from the sun has 

 been eliminated. In considering the average intensities actually meas- 

 ured at different seasons of the year, however, the effect of this variable 

 distance must not be overlooked. Thus, in early July the distance of the 

 earth from the sun is so much greater than in early January that with 

 the same air masses, other conditions being equal, early January solar- 

 radiation intensities should exceed July intensities by 7 per cent, while in 

 early April and early October the earth is at its mean distance from the 



Air Moiss 



Fig. 1. — Atmospheric transmission coefficients at various altitudes 



sun. As an illustration of the effect of this varying solar distance of the 

 earth, in Table 1 it is shown that at Washington, on August 22 and on 

 December 23, the measured noon solar-radiation intensity averages the 

 same, or 1.19 gm. cal./min./cm^. When reduced to mean solar distance, 

 however, the August intensity becomes 1.217, and the December inten- 

 sity 1.151, or 62.7 and 59.3 per cent, respectively, of the mean value of the 

 solar constant 1.94. 



In Table 1 are given the average measured hourly intensities of solar 

 radiation at normal incidence at five stations in the United States, on 

 dates when the sun is at its maximum south declination, —23° 27' on 

 December 23; on the equator, March 21 and September 23, farthest 

 north, -|-23° 27', on June 21, and halfway between the equinoxes and the 



